1. Field of the Invention
The present invention relates to feedback device and method for a constant current driver, and more particularly, to a feedback device and method with a negative feedback mechanism to allow a constant current driver to output a constant voltage to the load terminal to realize constant current driving.
2. Description of the Prior Art
Light emitting diodes (LEDs) are widely utilized in various fields such as backlight light sources of a display, outdoor and indoor illumination, visual display, light communication and infrared communications, due to their advantages of high energy conversion efficiency, low power requirements, small volume, fast response speed and long life, etc. Since the brightness of LEDs is related to the current flowing through the LEDs, a stable voltage should be applied to the LEDs (e.g., via a regulator), and a current source is implemented to provide a stable current for the LEDs, in order to allow the LEDs to output stable brightness.
However, there are always noises in an electronic device, such that the voltage and/or current outputted by the regulator and/or current source deviate from a predetermined value; hence, the current flowing through the LEDs may vary, resulting in unstable brightness. Therefore, a feedback mechanism is required to stabilize the voltage for driving the LEDs, in order to allow the current flowing through the LEDs to become stable. European Patent No. EP 1894300B1 discloses a circuit structure with multiple current sources, where a comparator is implemented to detect the voltage level on the output terminal, and voltage variations detected by the comparator may control the multiple current sources to be turned on or off, in order to control the regulator's output voltage to remain on a constant value. If the LEDs are applied as backlight sources of a display, the LEDs should output different brightness according to the color needing to be displayed. With different brightness requirements, the above current sources need to output a wide range of currents, e.g., 1 μA-128 μA. If one current source is used for realizing 1 μA current output, 128 μA current output will need 128 same current sources, which occupy a quite large layout area. In addition, all current sources in the large layout area cannot be gathered together. In such a situation, these current sources do not match and are easily influenced by process variation, such that there are differences generated between the output currents of every current source. This reduces the accuracy of output currents. When the currents from the 1st stage (1 μA) to the 128th stage (128 μA) have different rising degrees, there may be linearity errors and noises are easily generated in the load terminal, resulting in occurrence of blinks on the LEDs. Furthermore, the abovementioned linearity errors may also result in the problem of electromagnetic interference (EMI).
Thus, there is a need to provide another feedback mechanism to improve the abovementioned problems.